It was Williams' main passion project, dude worked on it by himself (and sometimes with fellow animators) for decades. Unfortunate that in the end he never got to finish it the way he wanted and the "finished" versions that were put out by companies like Miramax instead tried to turn the movie into a Disney knockoff (for one the movie wasn't supposed to have song numbers, and Tack was supposed to stay silent until the very end of the movie, and IIRC his one line was meant to be delivered by Sean Connery).

If anyone's interested in checking out a version of the movie that's closer to William's original vision then check the "Recobbled cut."

The character has no outline, but you can still make out the general shape based on the placement and movement of his head, arms, and sleeves. This is the kinda crazy shit you can only really get from 2D animation.